[esnext] Async function toString support

test/mjsunit/harmony/async-await-basic.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-async-await
 
 // Do not install `AsyncFunction` constructor on global object
 assertEquals(undefined, this.AsyncFunction);
 let AsyncFunction = (async function() {}).constructor;
 
@@ skipping 192 matching lines @@
     }
     try {
       value = await rejectLater("throw2");
     } catch (e) {
       log.push("resume:" + e);
     }
     return value + 1;`);
 log = [];
 assertEqualsAsync(5, () => resumeAfterThrowEval(4));
 assertEquals("start:4 resume:throw1 resume:throw2", log.join(" "));
+
+async function foo() {}
+assertEquals("async function foo() {}", foo.toString());